Although stereo sound has become universally accepted for sound reproduction, the exploitation of stereo effects in the field of amplified live music performance by solo performers and small groups has fallen short of its potential, even though many such performers are using instruments that provide capability of generating stereo output signals that could potentially enhance the overall effect of the musical performance.
The conventional practice for indoor stereo musical performance involves typically locating two spatially separated loudspeaker units at or near opposite ends of the stage or corners of the room, and driving each speaker with a dedicated amplifier, i.e. right and left channels. The physical separation between the two loudspeaker units is critical, depending on environmental factors such as the size and shape of the listening room, audience location, etc.
Excessive physical separation destroys the imaging performance, i.e. the "focus" of the point of origin of a sound image as perceived by a listener, e.g. an announcer, soloist or vocalist located at center stage where it most difficult for the two offset loudspeaker units to "synthesize" a focussed central image.
On the other hand, insufficient physical separation, while benefitting center imaging, degrades spatialization, i.e. the ability to create a full-width panoramic "sound stage" as perceived by the listener.
In practice, the limited physical separation selected as "optimal" is necessarily a tradeoff in which both imaging and spatialization are sacrificed. This "optimal" separation can perform acceptably for listeners located centrally, relative to the loudspeaker units. However, in most room or auditorium situations, some listeners will find themselves located too close to one of the loudspeaker units such that one channel will predominate: not only can the stereo effect be lost but in many cases the overall musical perception could even be judged as inferior to a monophonic performance of the same music. In the case where the musician is seeking to produce a highly spatialized stereo effect, a discriminating listener at an unbalanced listening location could perceive the performance as distracting or even totally unacceptable. The unbalance is also distracting to the musician if he is located too close to one of the stereo loudspeaker units.
If the musician or the sound technician attempts to mitigate this problem by placing the two speaker units closer together, or by utilizing a commercially available "combo-amp" unit having built-in forward-directed stereo speakers side by side built into a common case with the amplifier and thus having very little physical separation, this will tend to sacrifice the desired impact of the stereo effect, which could even become completely lost to all of the audience if the speakers are located so close together as to become practically monophonic.
While many keyboard and guitar players may develop special stereo effects in rehearsal and take great pains with these in recording, when it comes to live performance, in order to avoid ambivalence or controversy regarding stereo loudspeaker location, players may be reluctant to pursue excellent stereo performance and settle for an essentially monophonic loudspeaker setup such as a compact cluster or stack of speaker box units or the minimally separated "stereo" speakers of the "combo-amp". While this may result in a perception of more uniform coverage throughout the audience, there is a great sacrifice of the potential enhancement that effective stereo effects could add to their live performance.
In basic stereo recording modes, an orchestra can be recorded with a pair of identical microphones located at strategic symmetrical locations, e.g. at opposite sides of the orchestra stage facing inwardly to center or located centrally facing outwardly to opposite sides, and in the basic stereophonic system the two resulting independent signals L and R are customarily handled by a pair of independent channels throughout, including re-recording and playback in various media, broadcast, and all forms of reproduction equipment including power amplifiers and speakers. This basic L and R stereo system is in widespread common use as the standard system in home entertainment systems. Pure L and R signals may be subjected to some degree of cross-coupling alteration in master-recording and also in spatial-enhancement compensation circuity in small radios and stereo players having closely spaced built-in stereo speakers.
An alternative stereo recording technique known as "point source stereo" utilizes two different microphones centrally located close together, typically at front and center of the orchestra: one microphone is omnidirectional and the other is bidirectional (a.k.a. "figure 8" or "dipole") in directivity. The signals from the two microphones, being sum (L+R) and difference .+-.(L-R) respectively, can be handled in two audio channels, but they would normally be processed in a matrixing circuit to produce L and R stereo signals for conventional handling through L and R channels.
Though not currently in high volume usage, sum-difference loudspeaker systems have been proposed for home stereo entertainment centers, utilizing a front-firing speaker and a dual-side-firing dipole speaker, driven respectively by sum and difference signals derived by matrixing from standard L and R stereo signals.